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Fungal colonisation strategies Pt. IV: Active pathogenesis

Colonisation via active pathogenesis involves direct penetration of the host by the fungal pathogen, largely through the roots though also via air. The establishment of sufficient inoculum base (such as a dead stump or infected root) is critical for successful active pathogenesis, given the aggressive nature of active pathogenesis (Boddy & Rayner, 1983; Lonsdale, 1999; Schwarze et al., 2000; Shigo, 1986). Fungal species of this strategy can employ tactics that infect both healthy individuals and stressed individuals, depending both upon the species of fungus and the context of the site.

Active pathogenesis can be broken down into three categories: ectotrophic root infection, wound infection, and canker production (Boddy, 2001). Establishment is via the production of pectinolytic enzymes that destroy pit membranes and advance the spread of desiccated zones, or through wholly combative behaviour where parenchyma cells are destroyed completely during – or more likely in advance of – colonisation. The latter is achieved by the creation of superficial, predominantly non-assimilative mycelium (such as soil rhizomorphs with Armillaria spp.) that grows over the surface of roots, inducing dysfunction and cell death. In killing such cambial tissues, the fungi can colonise without significant hindrance (Garrett, 1970; Rayner, 1993; Rayner & Boddy, 1988).

Such strategists may also utilise pre-existing stress within the tree, caused for instance by defoliating insects or pathogenic disease, as a means of entry as a secondary pathogen. As energy must be used by the tree to combat the damage induced by such damaging agents, there is less energy available for additional defensive processes beyond that of combating the damaging agent. Active pathogenesis strategists can utilise this situation to their advantage, as it may mean that (when focussing on the rooting system of a tree) the boundaries between woody and non-woody roots do form form root periderms, become ‘corky’, or become suberised, leading to soil-borne fungal pathogens (such as Armillaria spp.) beginning their attack (Shigo, 1986). In some cases, such secondary infections can be so rapid that they are mis-identified as the primary causal agent.

An English oak (Quercus robur) that has been aggressively colonised (on all sides) by Armillaria mellea.

The well-known pathogens Heterobasidion annosum and (as ascertained) Armillaria mellea are classified as active pathogens, with colonisation of the sapwood being preceded by mycelial development in the bark. This leads directly to the death of the cambium within the region, and enables subsequent colonisation (Boddy & Rayner, 1983; Fox, 2000; Lonsdale, 1999; Schwarze et al., 2000). The preceding development in bark is a result of spores being washed into the soil, via rhizomorphs, or by direct contact with roots of a separate but infected host (Wargo & Shaw, 1985). Interestingly, Armillaria spp. will by-and-large colonise via soil rhizomorphs. This may be because the spores of the genus are suspected to have to pass through the guts of insects associated with the fungus, before they can successfully germinate (Shigo, 1986). Therefore, much like tree seeds, fungal spores may have an ‘activation’ process equivalent to stratification, fire, digestion, or otherwise – if the means of activation is not present, then the spores will not germinate.

As briefly touched upon above, infection can (perhaps only rarely) occur in branches when, under humid conditions, the fungus produces highly water-resistant bridges between branches that come within close proximity to one another. In the UK the fungus Hymenochaete corrugata, which is considered largely a specialised opportunist of Corylus avellana, establishes within the canopy and then spreads further by bridging from colonised canopy space to healthy branches of different hazel specimens (Ainsworth & Rayner, 1990).

Fungi that employ active pathogenesis as a means of colonisation may also rely initially upon the aforementioned colonisation strategies (heart rot, specialised opportunism, and unspecialised opportunism) in order to establish an inoculum base from which they can invade healthy sapwood. Stereum gausapatum is an example of this upon Quercus spp., where it is considered to exercise all four strategies to varying extents (Rayner, 1993).